Parabenzoquinone and hydroquinone are commonly used polymerization inhibitors. These compounds act as free-radical scavengers, mopping up any free radicals formed in the monomer solution before they initiate polymerization. The two compounds are in equilibrium in a protic solvent: ##STR1## Parabenzoquinone is known to react with an aliphatic double bond and oxygen on exposure to the 488 .mu.m output line of a one-watt continuous wave argon ion laser (see article by I. R. M. Wilson and S. M. Wunderly in the Journal of the American Chemical Society, 1974, 96, 7350): ##STR2##